It’s worth making the point here that we don’t get an objective reading of everything we sniff. Our senses are about interpretation and context as much as they are about chemicals. As a result, psychological factors will play in to our perception of how ‘bad’ a fart smells, along with the details of last night’s dinner menu. So, even if the chemical composition of ‘our’ fart and ‘their’ fart is the same, we may still perceive them as smelling different.
1. You’re used to the smell of your own brew. Both consciously and subconsciously, you smell yourself more than pretty much anything else in the world. A proportion of the gases made by your intestines dissolve into your blood and get breathed out; it’s not a large or concentrated amount, but it means your sense of smell will be more used to what ‘flavours’ your body has made than other people’s. This also serves to explain why people rated the smell of their ‘chosen person’ as less disgusting than a stranger.
2. When you deal it out, you’re prepared. Being ready for a sensation can make it easier to cope with. For example, we find it almost impossible to tickle ourselves. This might apply to flatulence as well. However, it doesn’t explain the situation when you hear a stranger let one rip. You know something bad is on its way, but that does nothing to lessen the noxiousness when it arrives.
3. It’s more dangerous coming from someone else. This is the currently favoured evolutionary explanation. It’s grosser to smell, see or otherwise experience the bodily functions of other people, because they pose more of a disease risk. Over thousands of years, our ancestors who steered clear of such things were likely to stay healthier, and passed on that innate, psychological ‘yuck’ reflex to us. Think about an example: if you sneeze into your hands, it’s a bit icky, but OK. If a complete stranger on the train leaned over and sneezed onto your hand, that would be WRONG.
4. It’s socially conditioned. There’s a social convention at work here, too. Flatulence is an intrusion. If a stranger intrudes on your experience of the world in an unpleasant way, of course we’re going to react badly!
5. Confirmation bias. We will only really notice a bad-smelling fart. Most people fart multiple times per day, but not all will smell bad (diet etc come into play here). Se we figure, on average, we aren’t too bad. In public, we only smell the bad ones – confirming our suspicion that everyone else smells worse then we do!